Every Which Way but Loose
(1978)

Philo Beddoe (Clint Eastwood) finally tracks down Lynn (Sondra Locke), but is heartbroken to learn that she doesn't care about him, and has been trying to avoid him.

Later, Philo takes on Tank Murdock in a big stakes bare knuckle fight. Philo gives Murdock a severe beating, but when he hears spectators proclaiming him "the new Tank Murdock," he looks at the bloodied Murdock and decides he doesn't want that kind of life. He takes a dive, and lets Murdock win.

As he drives his pickup truck back to Bakersfield, he sees the hapless Black Widow motorcycle gang he'd been abusing all through the movie, and laughs.

Thanks John L!